Scientists unveil the mystery of the ‘slow’ solar wind with solar orbiter mission

ESA Solar Orbiter. Credit: European Space Agency (ESA)

Scientists have made significant progress in understanding the mysterious origins of the “slow” solar wind, thanks to data collected during the Solar Orbiter spacecraft’s first close journey to the sun.

This breakthrough sheds light on how this solar wind forms and behaves.

The solar wind is a continuous outflow of charged plasma particles from the sun into space.

It can travel at speeds of hundreds of kilometers per second, with wind traveling over 500 km/s called ‘fast’ and under 500 km/s referred to as “slow.”

When the solar wind hits Earth’s atmosphere, it can create stunning auroras, such as the Northern Lights. However, larger plasma releases, known as coronal mass ejections, can be hazardous, causing damage to satellites and communication systems.

Despite decades of research, the origins and mechanisms behind the solar wind, particularly the slow solar wind, have remained elusive.

In 2020, the European Space Agency (ESA), supported by NASA, launched the Solar Orbiter mission to help answer these questions. The mission’s goal is to measure and link the solar wind back to its origin on the sun’s surface.

The Solar Orbiter, described as “the most complex scientific laboratory ever sent to the sun,” carries ten different scientific instruments.

Some of these instruments collect and analyze solar wind samples as they pass the spacecraft, while others capture high-quality images of the sun’s surface.

By combining photographic and instrumental data, scientists have identified where the slow solar wind originates.

This has helped them understand how the slow wind leaves the sun and travels into the heliosphere, the bubble around the sun and its planets that protects our solar system from interstellar radiation.

Dr. Steph Yardley from Northumbria University led the research. She explains, “The variability of solar wind streams measured close to the sun provides us with information about their sources. Previous studies traced the solar wind’s origins much closer to Earth, by which time this variability is lost.

Because Solar Orbiter travels so close to the sun, we can capture the complex nature of the solar wind and get a clearer picture of its origins.”

The difference in speed between the fast and slow solar wind is thought to be due to the different areas of the sun’s corona, the outermost layer of its atmosphere.

The open corona, with magnetic field lines stretching into space, allows solar material to escape easily and is the source of the fast solar wind.

The closed corona, with magnetic field lines connected to the sun at both ends, occasionally allows material to escape when these lines break and reconnect.

One aim of the Solar Orbiter mission is to test the theory that the slow solar wind originates from the closed corona. The scientific team measured the composition of solar wind streams, noting that the mix of heavy ions in solar material differs depending on its origin.

Using the Solar Orbiter’s instruments, the team analyzed the sun’s surface activity and matched it with the solar wind streams collected by the spacecraft.

They pinpointed that the slow wind streams came from areas where the open and closed corona meet, supporting the theory that slow wind escapes through magnetic reconnection.

Dr. Yardley says, “The varying composition of the solar wind measured at Solar Orbiter was consistent with changes in composition across the corona’s sources. This provides strong evidence that the variability is driven by different source regions and reconnection processes in the corona.”

The Solar Orbiter mission is an international collaboration with scientists and institutions worldwide contributing specialist skills and equipment. Daniel Müller, ESA Project Scientist for Solar Orbiter, states, “This result confirms that Solar Orbiter can robustly connect the solar wind to its source regions on the solar surface, opening the way for us to study the solar wind’s origin in unprecedented detail.”

Looking ahead, Dr. Yardley notes that future research will involve analyzing more data from Solar Orbiter and comparing it with other close-in missions like NASA’s Parker Solar Probe.

This ongoing research promises to deepen our understanding of the sun and its impact on our solar system.